1. Field of the Invention
The present invention relates to a hologram and an exposure apparatus.
2. Description of the Related Art
A projection exposure apparatus has conventionally been employed to fabricate a micro-patterned semiconductor device such as a semiconductor memory or a logic circuit by using photolithography (printing). The projection exposure apparatus projects and transfers a circuit pattern formed on a reticle (mask) onto a substrate such as a wafer via a projection optical system.
The resolution R of the projection exposure apparatus is given by:
                    R        =                              k            1                    ×                      λ            NA                                              (        1        )            where λ is the exposure light wavelength, NA is the numerical aperture of the projection optical system, and k1 is a process constant determined by, for example, a development process.
The shorter the exposure-light wavelength or the higher the NA of the projection optical system, the higher the resolution. However, it might be difficult to further shorten the current exposure light wavelength because the transmittance of a glass material generally decreases as the exposure-light wavelength shortens. It might also be difficult to further increase the NA of the projection optical system available at present because the depth of focus decreases in inverse proportion to the square of the NA of the projection optical system, and because it might be difficult to design and manufacture lenses to form a high-NA projection optical system.
The application of resolution enhanced technologies (RETS) have been proposed for improving the resolution by decreasing the process constant k1. One of these RETS is the so-called modified illumination method (oblique or off-axis illumination method).
The modified illumination method generally inserts an aperture stop, which has a light-shielding plate on the optical axis of an optical system, in the vicinity of the exit surface of an optical integrator that forms a uniform plane illumination, thereby obliquely irradiating a reticle with exposure light.
The modified illumination method includes, for example, an annular illumination method and a quadrupole illumination method that are different in the aperture shape of an aperture stop (i.e., the shape of the light intensity distribution). There has also been proposed another modified illumination method which uses a computer generated hologram (CGH) in place of an aperture stop in order to improve the use efficiency (illumination efficiency) of the exposure light.
Along with an increase in the NA of the projection optical system, a polarized illumination method which controls the polarization state of exposure light is also required to increase the resolution of the projection exposure apparatus. The polarized illumination method illuminates a reticle with, for example, S-polarized light alone, which has an electric field component in the circumferential direction about the optical axis. The contrast of the image to be formed might be enhanced by using S-polarized light alone.
In recent years, there has been proposed a technique which exploits both the modified illumination method (the formation of a light intensity distribution having a desired shape, e.g., a quadrupolar shape) and the polarized illumination method (i.e., polarization state control).
For example, Japanese Patent Laid-Open No. 2006-196715 discloses a technique which implements both the modified illumination method and polarized illumination method using a light beam conversion element composed of a variety of combinations between a form birefringence region and a diffraction region. Japanese Patent Laid-Open No. 2006-196715 describes controlling the polarization state using a form birefringence region and the shape (i.e., a reconstructed image) of the light intensity distribution at a predetermined plane using the diffraction region. The number of combinations depends on the kinds of polarization states formed on the predetermined plane.
U.S. Pat. No. 7,265,816 (or Japanese Patent Laid-Open No. 2006-5319) discloses a technique which can control the balance among the four poles of a quadrupolar light intensity distribution typically formed by the modified illumination method and the polarized illumination method. U.S. Pat. No. 7,265,816 refers, after converting four circularly polarized lights into four linearly polarized lights different from each other with a quarter wave plate (QWP), to changing the light intensity distribution at a predetermined plane by controlling the balance with using four separated CGHs which function as a diffractive optical element corresponding to each linearly polarized light.
A CGH design technique is disclosed in “Iterative method applied to image reconstruction and to computer-generated holograms”, OPTICAL ENGINEERING, Vol. 19, No. 3, May/June 1980, 297-305. The conventional technique uses a plurality of the separated CGHs to form a reconstructed image composed of a plurality of polarization states, and the number of separated CGHs depends on the number of a variety of polarization states.
When the plurality of the separated CGHs combined with each other is used, an irradiance variation might occur in a reconstructed image. If an optical integrator cannot sufficiently correct the intensity distribution of the incident light, the light may impinge on only some of the plurality of CGHs.